Request Sync Manager & V2 Packet Updates

This document details the implementation of the Request Sync Manager and updates to the V2 packet structure to improve synchronization security and attribution on iOS, mirroring the Android implementation.

Overview

The goal of these changes is to make the request sync functionality "less blind". Previously, sync requests were broadcast, and responses were accepted without strict attribution or timestamp validation (to allow syncing old messages). This opened up potential spoofing vectors and prevented us from enforcing timestamp checks on normal traffic.

The new implementation introduces a RequestSyncManager to track outgoing sync requests and attributes incoming responses (RSR - Request-Sync Response) to specific peers. This allows us to:

1. Enforce Timestamp Validation: Normal packets now require timestamps to be within 2 minutes of the local clock.
2. Exempt Solicited Sync Responses: Packets marked as RSR are exempt from timestamp validation only if they correspond to a valid, pending sync request sent to that specific peer.
3. Prevent Unsolicited Sync Floods: Unsolicited RSR packets are rejected.

Protocol Changes

Binary Protocol Updates

• New Flag: IS_RSR (0x10) added to the packet header flags.
• BitchatPacket: Updated to include isRSR: Bool field.
• Encoding/Decoding: Updated BinaryProtocol to handle the new flag.

Request Sync Payload

The REQUEST_SYNC packet payload (TLV encoded) has been updated to include:

• sinceTimestamp (Type 0x05): filter-coverage cursor (UInt64 big-endian). The requester's GCS filter only covers packets at or after this timestamp; the responder skips older packets instead of re-sending them every round.
• fragmentIdFilter (Type 0x06): targeted fragment resync (UTF-8 string). Comma-separated 16-hex-char (8-byte) fragment stream IDs — the ID that prefixes every fragment payload.
  • Requester: when a broadcast reassembly stalls (no new fragment for 5 s), the fragment assembler reports the stream ID and a REQUEST_SYNC with types = fragment and this filter goes to each connected peer (re-requested at most every 10 s per stream). Directed reassemblies are excluded — peers only archive broadcast fragments for sync.
  • Responder: when the filter is present, the fragment diff is restricted to exactly the named streams and the sinceTimestamp cursor is bypassed for them; the GCS filter still excludes pieces the requester already holds. Responses keep RSR marking, TTL 0, per-peer response rate limiting (8/30 s), and REQUEST_SYNC itself remains link-local (TTL 0, never relayed).
  • Bounds: at most 60 IDs per request. Each ID encodes as 16 hex chars plus a comma separator, so the largest value is 60 × 17 − 1 = 1019 bytes, within the decoder's 1024-byte acceptance cap; oversized filter values are ignored (the rest of the request still decodes).

Architecture

RequestSyncManager

A new component (Sync/RequestSyncManager.swift) responsible for:

• Tracking: Stores peerID -> timestamp mappings for pending sync requests.
• Validation: isValidResponse(from: PeerID, isRSR: Bool) checks if an incoming RSR packet matches a pending request within the 30-second window.
• Cleanup: Periodically removes expired requests.

GossipSyncManager Updates

• Unicast Sync: Instead of blind broadcasting, the periodic sync task now iterates over connected peers and sends unicast REQUEST_SYNC packets.
• Registration: Before sending, requests are registered with RequestSyncManager.
• Response Marking: When responding to a REQUEST_SYNC, generated packets (Announce/Message) are explicitly marked with isRSR = true (and ttl = 0).

BLEService (Security Manager) Updates

• Timestamp Enforcement: Checks abs(now - packetTimestamp) < 2 minutes for standard packets.
• Conditional Exemption: If packet.isRSR is true (or packet is a legacy TTL=0 response), it queries RequestSyncManager.
  • Valid: If solicited, timestamp check is skipped (allowing historical data sync).
  • Invalid: If unsolicited or timed out, the packet is rejected.

Usage

These changes are integrated into BLEService and GossipSyncManager. No external API changes are required for clients, but all peers must be updated to support the new IS_RSR flag and protocol logic to participate in the secure sync process.